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I have a Windows 10 KVM VM on Ubuntu 20.04. The host has dual 10 GbE adapters configured as LACP bond and there's a bridge interface for the VM and it's on the same LAN as the host.

Windows has VirtIO driver version 100.77.104.17100.

Testing with iperf from the Ubuntu host to another host on the LAN I can get over 8 Gbps without any tuning.

But running iperf between the Windows VM and the Ubuntu host, so it doesn't even leave the box, I get only about 1 Gbps, sometimes short bursts little over that. If I run parallel iperfs the aggregate is about 2.4 Gbps.

What might be the bottleneck here? Driver issue perhaps or some tuning required?

nixer
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  • Can you share how the virtual network between host & guest is setup? Do you see a high increase in CPU load while running iperf? – eKKiM Apr 12 '21 at 11:19
  • Hi @eKKiM. The two NIC's are in bond0 but the host IP address is in br0 bridge interface. This was done with Netplan. There's a virbr0 interface but that shouldn't be really doing anything. There's also a vnet0 interface which seems to be for the guest VM. I'm not that familiar with KVM so I followed a guide to set this up for the single VM that was needed. Iperf doesn't really cause any significant increase in CPU usage, but the guest VM is idling at 100% which seems to be common from what I read. – nixer Apr 13 '21 at 07:34
  • @eKKiM I noticed now that inbound traffic to the VM seems to be faster than outbound. Inbound I get up to 2 Gbps. – nixer Apr 14 '21 at 08:55

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